MicroRNAs (miRNAs) appear to play a role in mediating interactions between cancer cells and their hosting niche during cancer progression and metastasis. Our preliminary studies indicate that miR-105, whose levels in the circulation are associated with metastatic progression in early-stage breast cancer (BC) patients, is characteristically expressed and secreted by metastatic BC (MBC) cells. MiR-105 downregulates the tight junction protein ZO-1, for which reduced expression is associated with metastasis in BC patients. MiR-105 potently induces migration and proliferation in MBC cells, and can be transferred via MBC-secreted exosomes to normal epithelial and endothelial cells in the cancer niche, where it alters ZO-1 expression and the barrier function of these niche cells. In a mouse model established in our lab, MBC-secreted exosomes and miRNAs can be internalized by cells in various tissues, and can facilitate metastasis development. The goals of this study are to dissect the dual roles of miR-105 in regulating the metastatic potential of cancer cells and in destroying the epithelial and endothelial barriers in the cancer niche, and t explore novel therapeutic strategies that target miR-105-mediated pro-metastatic functions. In Aim 1, the effects of miR-105 on cancer cell adhesion, migration, invasion, proliferation and anchorage-independent growth, as well as the role of ZO-1 in mediating these effects, will be investigated. Additional miR-105-regulated genes will be identified and their role in mediating miR-105's effects will be determined. In Aim 2, the effects of cancer-secreted, exosome-transferred miR-105 on normal epithelial and endothelial niche cells will be determined, focusing on their barrier functions to restrict cancer cell invasion and metastasis. The magnitude and kinetics of miR-105-mediated barrier-destroying effects will be determined by co-culturing the epithelial and endothelial niche cells with MBC cells that secrete miR-105. In Aim 3, the in vivo effects of miR-105 on niche adaptation and BC metastasis will be determined using mouse xenograft models of BC. The anti-metastatic effect of miR- 105 intervention will be evaluated. These in-depth functional studies of cancer-secreted miRNAs that contribute to the co-evolution of the tumor-hosting environment will provide novel insights into the dynamic communication between cancer and host during disease progression. This study will also provide proof-of- principle for targeting cancer-secreted miRNAs as a novel approach to block the cancer-directed, pro-metastatic remodeling of the niche at early cancer stages for the prevention of metastasis. Our long-term objectives are to validate the miR-105 pathway in primary BC and establish standard approaches to identify patients suitable for therapies that target miR-105, to understand the global effects of cancer-secreted miRNAs, and to elucidate cancer-secreted, circulating miRNAs (e.g., miR-105) in BC patients as blood-borne markers for early diagnosis or prediction of metastasis.